In carrying out broadcast wave relay SFN (Single Frequency Network), for example, in terrestrial digital broadcasting, use of an OFDM transmission scheme is under study in recent years. The OFDM transmission scheme is a scheme whereby many carriers orthogonal to one another are modulated by digital data transmitted, those modulated signals are multiplexed and transmitted. The OFDM transmission scheme is characterized in that increasing the number of carriers used to several hundreds or several thousands extends the symbol time extremely and further adding a replica of a signal of the last part of an effective symbol period before the effective symbol period as a guard period signal makes the signal less susceptible to delay signals.
These features raise the possibility of a broadcasting network through a single frequency, that is, SFN, and therefore as described above, the OFDM transmission scheme is attracting attention as a transmission scheme for terrestrial digital broadcasting.
As a method of realizing an SFN, it is technologically easier to adopt a method of transmitting a signal to various relay broadcasting points using channels different from channels for broadcasting waves such as optical fibers and microwave and transmitting the signal at the same frequency. However, the method using optical fibers has a problem of channel cost and the method using microwave requires new frequency resources to be secured.
Therefore, there is a demand for the realization of an SFN through relays of broadcast wave without requiring additional frequency resources.
Realizing a broadcast wave relay SFN involves the possibility of causing deterioration of the quality of the relay signal and problems of oscillation, etc., of an amplifier because of a phenomenon of radio waves emitted from a transmission antenna wrapping around a reception antenna.
To prevent loop interference of the broadcast wave relay SFN, the following measures may be possibly taken:
(1) Reduce loop interference by arranging a transmission antenna separate from a reception antenna and using shielding by mountains and buildings, etc.
(2) Reduce loop interference by improving directional characteristics of transmission/reception antennas
(3) Cancel loop interference by signal processing technology
However, since there are various situations of mountains and buildings and sufficient suppression of loop interference cannot be expected from measures like improvement of directional characteristics of antennas alone, it is effective to use a loop interference canceller using the signal processing technology in (3) in addition to the measures in (1) and (2).
Conventionally, there is a proposal on a technique as such a signal processing technology (e.g., see Unexamined Japanese Patent Publication No. 11-355160) which estimates a frequency characteristic of a loop interference transmission path from a received OFDM signal, carries out an IFFT (Inverse Fast Fourier Transform) on the frequency characteristic data of the estimated loop interference transmission path to transform it into impulse response data on the time axis, sets the impulse response data as filter factors in a transversal filter to thereby create a replica signal of the loop interference and cancel the loop interference by subtracting this replica signal from the received signal. Furthermore, as its high-speed calculation processing technology, there is a transmission path characteristic estimation section provided with a decimating processing circuit (e.g., see Unexamined Japanese Patent Publication No. 2001-223663). An example of the signal processing technology in a loop interference canceller will be explained using a drawing.
FIG. 1 is a schematic view of an example of an arrangement of pilot signals and shows an arrangement of pilot signals used in DVB-T (Digital Video Broadcasting- Terrestrial) scheme which is a European terrestrial digital broadcasting scheme and ISDB-T (Integrated Services Digital Broadcasting-Terrestrial) scheme which is a Japanese terrestrial digital broadcasting scheme.
White circles in FIG. 1 represent data carriers and black circles represent pilot carriers (SP: Scattered Pilot) which are scatteringly arranged.
Furthermore, in FIG. 1, k on the horizontal axis (frequency axis) denotes a carrier number and n on the vertical axis (time axis) denotes a symbol number. At this time, an SP signal is transmitted using a carrier having a carrier number k=kp that satisfiers the following (Expression 1). (Where, “mod” in the expression denotes a calculation of a remainder and “p” denotes a non-negative integer.)kp=3(n mod 4)+12p  (Expression 1)
From this (Expression 1), it is clear that the carrier number of the SP is determined by the remainder of 4 of the symbol number n.
Furthermore, the SP signal is modulated based on a pseudo-random code string, the amplitude and phase thereof are determined by only the carrier number k arranged and do not depend on the symbol number n. How to determine the amplitude and phase is not important to this explanation and will be omitted, but it is determined by the remainder of 4 of the symbol number n as in the case of the carrier number of the SP.
Furthermore, at the right end of carriers, pilot signals are placed independently of symbol numbers. This pilot signal is also modulated based on a pseudo-random code string and the amplitude and phase thereof are determined by the remainder of 4 of the symbol number n. When the remainder of 4 of the symbol number n is 0, this pilot signal also follows (Expression 1), and therefore hereafter, pilot signals will also be included in the definition pilot carriers or SP.
FIG. 2 is a block diagram showing a configuration example of a loop interference canceller 3.
Inside a filter factor generation section 33, a transmission path characteristic estimation section 331 estimates a transmission path characteristic F(ω) from an output s(t) of a subtractor 31 and the output is supplied to an input of a residual characteristic calculation circuit 3309.
Inside the transmission path characteristic estimation section 331, an FFT (First Fourier Transform) circuit 3301 extracts a signal whose length corresponds to an effective symbol period from the output s(t) of the subtractor 31, applies an FFT to thereby transform s(t) which is a signal in a time domain into a signal in a frequency domain and the output s(ω) is supplied to an input of a symbol number extraction circuit 3302 and a first input of an SP extraction circuit 3303.
The symbol number extraction circuit 3302 extracts a symbol number for specifying the carrier number of SP from information on symbols such as TMCC (Transmission Multiplexing Configuration Control) included in the input s(ω). After symbol numbers are extracted once, adding the symbol numbers may also substitute for extraction processing. The remainder of 4 of the symbol number which is minimum information necessary to specify the carrier number, amplitude and phase of SP is output and the output is supplied to the respective second inputs of the SP extraction circuit 3303, a transmission path characteristic calculation circuit 3304 and an SP combination circuit 3305. Hereinafter, the symbol number will no longer be used directly, and therefore the remainder of 4 of the symbol number will be renamed a “symbol number.”
According to the specification of the symbol number extraction circuit 3302, the SP extraction circuit 3303 extracts a signal Sp(ω) of only the SP signal from the output s(ω) of the FFT circuit 3301 and the output Sp(ω) is supplied to a first input of a transmission path characteristic calculation circuit.
According to the specification of the symbol number extraction circuit 3302, the transmission path characteristic calculation circuit 3304 generates a specified SP signal Xp(ω) whose amplitude and phase are known inside, divides the SP signal Sp(ω) which is the output of the SP extraction circuit 3303 by the Xp(ω) to calculate a transmission path characteristic Fp(ω) with respect to SP and the output is supplied to a first input of the SP combination circuit 3305.
The SP combination circuit 3305 stores transmission path characteristic Fp(ω) corresponding to four symbols for the SP, combines the SP distributed to the four symbols into the original arrangement of carriers according to the instruction of the symbol number extraction circuit 3302 and newly outputs a transmission path characteristic Fp′(ω) corresponding to the combined SP. That is, it rearranges carriers in order of left end of Fp(ω) with symbol number 0, left end of Fp(ω) with symbol number 1, left end of Fp(ω) with symbol number 2, left end of Fp(ω) with symbol number 3, second from the left end of Fp(ω) with symbol number 0, . . . . The transmission path characteristic Fp′(ω) with respect to the combined SP which is the output is supplied to interpolation circuit 3306.
The interpolation circuit 3306 interpolates the transmission path characteristic Fp′(ω) scatteringly obtained with respect to the combined SP and estimates a transmission path characteristic for the entire signal band.
That is, the interpolation circuit 3306 interpolates the transmission path characteristic at the positions of data carriers deleted from between SPs and obtains a transmission path characteristic for the entire signal band using the transmission path characteristic for the already calculated SP. There may be various possible methods for interpolation such as a method of applying a low pass filter in the carrier direction. This method realizes interpolation by carrying out a convolutional calculation according to an impulse response of the low pass filter. However, from the standpoint of accuracy and stability, the impulse response cannot be helped but be set to a finite length. The interpolation circuit 3306 outputs the transmission path characteristic for the entire signal band obtained and the output is supplied to a decimating circuit 3308.
The decimating circuit 3308 decimates data and reduces the number of data pieces to shorten the processing time in the subsequent circuits. The decimating processing is carried out in such a way as to prevent the phase relationship from being shifted by the transform into the time axis at the IFFT circuit 3310 and prevent the position of carrier data which becomes a central frequency of IFFT processing from being shifted. Under the constraint of IFFT processing, data is decimated for every power of 2 in the number of data pieces. The number of data pieces decreases as the decimating interval increases, but as described in the Unexamined Japanese Patent Publication No. 2001-223663, there is a practical limit and it is limited to approximately 2 or 4. Furthermore, when data is not decimated, the decimating circuit can be omitted. The data F(χ) after the decimating is output from the transmission path characteristic estimation section 331 and the output is supplied to the residual characteristic calculation circuit 3309.
FIG. 3 schematically expresses the internal operation of the transmission path characteristic estimation section 331. The operation has already been explained, and therefore the figure will be used only for reference and explanations thereof will be omitted.
The residual characteristic calculation circuit 3309 calculates a cancellation residual E(ω) from the output F(ω) of the transmission path characteristic estimation section 331 and the output is supplied to the IFFT circuit 3310.
The IFFT circuit 3310 carries out an IFFT on the output E(ω) of the residual characteristic calculation circuit 3309 to thereby transform a residual E(ω) in the frequency domain into a residual e(t) in the time domain and the output is supplied to a factor updating circuit 3311.
The factor updating circuit 3311 calculates a filter factor w_new(t) from the output e(t) of the IFFT circuit 3310 based on a predetermined factor updating expression and the output is supplied as the output w_fir(t) of the filter factor generation section 33 to a second input of a FIR filter 32.
Then, the condition under which the loop interference canceller 3 cancels loop interference will be explained.
First, the output F(ω) of the transmission path characteristic estimation section 331 is expressed by (Expression 2).
                              F          ⁡                      (            ω            )                          =                              w_in            ⁢                          (              ω              )                                            1            -                          {                                                w_in                  ⁢                                      (                    ω                    )                                    ⁢                  w_out                  ⁢                                      (                    ω                    )                                    ⁢                  w_loop                  ⁢                                      (                    ω                    )                                                  -                                  w_fir                  ⁢                                      (                    ω                    )                                                              }                                                          (                  Expression          ⁢                                          ⁢          2                )            
Therefore, the condition under which the loop interference signal is canceled by the subtractor 31 is expressed by (Expression 3).w—in(ω)w—out(ω)w—loop(ω)=w—fir(ω)  (Expression 3)
Here, when the cancellation residual E(ω) is defined as shown in (Expression 4),E(ω)=w—in(ω)w—out(ω)w—loop(ω)−w—fir(ω)   (Expression 4)to modify (Expression 2), then (Expression 5) is obtained.
                              E          ⁡                      (            ω            )                          =                  1          -                                    w_in              ⁢                              (                ω                )                                                    F              ⁡                              (                ω                )                                                                        (                  Expression          ⁢                                          ⁢          5                )            
Here, suppose the frequency characteristic of the reception section is flat within the signal band using a simplified model. The transfer function w_in(ω) becomes constant D and is calculated within the residual characteristic calculation circuit 3309 based on (Expression 6).
                    D        =                              ∑            ω                    ⁢                      F            ⁡                          (              ω              )                                                          (                  Expression          ⁢                                          ⁢          6                )            
At this time, the cancellation residual E(ω) is expressed by (Expression 7).
                              E          ⁡                      (            ω            )                          =                  1          -                      D                          F              ⁡                              (                ω                )                                                                        (                  Expression          ⁢                                          ⁢          7                )            
Furthermore, the factor updating expression at the factor updating circuit 3311 is defined by (Expression 8)w—new(t)=w—old(t)+μe(t)  (Expression 8)where, w_old(t) in (Expression 8) is a factor before updating and μ is a non-negative constant of 1 or below.
In the above described configuration, feedback control operates so that the cancellation residual E(ω) which is a difference between the loop interference transfer function w_loop(ω)w_out(ω) and transfer function w_fir(ω) of the FIR filter 32 converges to 0 and only the key station wave component is output to the output s(t) of the loop interference canceller 3.
FIG. 4 is a block diagram in which annotations are made on the number of data pieces processed at various sections of the loop interference canceller 3. Connections in the respective sections and their processing are completely the same as those in FIG. 2, and therefore explanations of their operations will be omitted. The number of data pieces applies to the case of the transfer in mode 3 of the aforementioned ISDB-T scheme.
At the input/output of the FFT circuit 3301, the input of the symbol number extraction circuit 3302 and the first input of the SP extraction circuit 3303, the number of data pieces is 8192. At the output of the SP extraction circuit 3303, the first input and output of the transmission path characteristic calculation circuit 3304 and the first input of the SP combination circuit 3305, the number of data pieces is 469 which is the number of SPs included in one symbol. At the output of the SP combination circuit 3305 and the input of the interpolation circuit 3306, the number of data pieces is 1873 which is the number of SPs corresponding to four symbols (however, the pilot at the right end is common). At the output of the interpolation circuit 3306 and the input of the decimating circuit 3308, the number of data pieces represents a carrier allocation, and therefore it is 8192 in the same way as the input/output of the FFT circuit 3301. At the output of the decimating circuit 3308, the input/output of the residual characteristic calculation circuit 3309 and the input/output of the IFFT circuit 3310, the number of data pieces changes according to how data is reduced by the decimating processing, but it is the same number of data pieces and realistically 2048 or 4096 or 8192.
This loop interference canceller is required to provide high trackability with respect to time variations of the phase and level of a loop interference wave or key station wave, high accuracy cancellation operation and miniaturization of the apparatus.
However, in the above described configuration, the transmission path characteristic of the entire signal band is estimated through interpolation on the transmission path characteristic by SPs first and then the residual characteristic is calculated, and therefore the number of data pieces increases, which prevents high-speed processing, and moreover the incompleteness of the impulse response of a low pass filter used for interpolation (e.g., finite length) reduces the estimation accuracy of the transmission path characteristic and there is also a problem that the low pass filter requires a large circuit scale.